1. Field of the Invention
The disclosure generally relates to a mobile TV system, and a method for synchronizing the rendering of a streaming service in a plurality of user terminals.
2. Description of Related Art
Currently, as for streaming services provided by an IP-Based mobile TV system, time-based A/V streams, interactive services, or notification service are transmitted to a user terminal device of a mobile TV through a Real-time Transport Protocol (RTP) in a broadcast or multicast manner.
In particular applications, the same streaming service may need to be displayed synchronously on mobile TV user terminal devices. For example, when the streaming service includes an interactive service (such as real-time callback and real-time question answering), in order to ensure fairness of the interactive service, the time of rendering the streaming service on different mobile TV user terminals has to be synchronized. Alternatively, when users of the mobile TV user terminals need to watch the same streaming service and talk with each other about the content of the streaming service through the respective user terminal device at the same time, if the rendering of the streaming service is synchronized on the user terminals, it will facilitate the discussion. Moreover, a plurality of mobile TV user terminal devices may be placed in one location for demonstration or for sale at the same time. At this time, if the mobile TV user terminal devices are able to receive and render the same streaming service synchronously, the customers are able to compare the differences of the display characteristics on the mobile TV user terminal devices conveniently.
For a conventional analog and digital TV, it is not a problem to display the same program synchronously. However, for an IP-based mobile TV system, for example, Digital Video Broadcasting-Handheld (DVB-H), 3rd Generation Partnership Project Multimedia Broadcast Multicast Service (3GPP MBMS), Worldwide Interoperability for Microwave Access Multicast Broadcast Service (WiMAX MBS), since the RTP is originally designed for a unicast streaming service, a problem of synchronization of rendering the same streaming service on different mobile user terminal devices in broadcast or multicast transmission is not taken into consideration.
FIG. 1 is an exemplary schematic view of transmitting a multimedia stream through RTP according to the related art. The multimedia stream includes an audio stream, a video stream, and an event stream.
Referring to FIG. 1, time stamps TSa(i), TSv(i), and TSe(i) are RTP time stamps used on the audio stream, the video stream, and the event stream transmitted through RTP respectively. The clock rates of the RTP time stamps are essentially the sample rates of the audio stream, the video stream, and the event stream, respectively. In addition, the synchronization information required for rendering the audio stream, the video stream, and the event stream at the same time is carried by Real-time Transport Control Protocol (RTCP) packets. Specifically, each media stream (that is, the audio stream, the video stream, and the event stream) has a dedicated RTCP channel. The RTCP channel is responsible for transmitting an RTCP Sender Report (SR) packet required by the corresponding media stream periodically. One RTCP SR packet includes a Network Time Protocol (NTP) time stamp and a corresponding RTP time stamp of the media stream. For example, the NTP time stamp corresponding to the RTP time stamp Vt(1) of the video stream is NTPv(1), and the NTP time stamp corresponding to the RTP time stamp At(1) of the audio stream is NTPa(1). As the NTP time stamps included in the RTCP SR packets of the media streams come from the same wall clock, a plurality of media streams may be rendered synchronously on one user terminal device.
However, in an IP-Based mobile TV system, the NTP time stamps of the RTCP SR packets do not record the exact time for an A/V multimedia stream to be actually rendered on a mobile TV terminal. In fact, the wall clock and the time recorded by the NTP time stamps are not strictly defined, and usually an NTP time stamp contains the instance of time when a plurality of media streams is synchronously mixed into a multimedia stream. Moreover, in IP-Based mobile TV systems, no standard method is specified to determine the corresponding relationship between the clock recorded by the NTP time stamps in the RTCP SR packets and the clock on that the multimedia stream is actually rendered. Therefore, due to performance differences in mobile user terminal devices, different initial delays for rendering will occur when the same streaming service is received for rendering. This causes the phenomenon of asynchronous rendering of the streaming service. Taking the DVB-H as an example, the Multiple-Protocol Encapsulation-Forward Error Correction (MPE-FEC) decoding delay in the Media Access Control (MAC) layer and the initial buffering delay of an AV decoder are both the causes of the differences in initial delays for streaming rendering on different mobile TV user terminals. Therefore, it is necessary to develop a system that is capable of synchronizing the rendering of a streaming service on a plurality of mobile TV user terminals in an IP-Based mobile TV system.